1. Field of the Invention
This invention relates in general to an image output device, and more particularly to digital halftoning techniques for controlling a spot function.
2. Description of Related Art
To print an image, a print engine processor, referred to herein as a raster image processor, converts the image in a page description language to a bit mapped image indicating a value to print at each pixel of the image. The bit mapped image is sent to the printer to cause the print heads to print the specified color value at the pixel according to the information in the bit map. If a printer has multiple print heads, such as a print head for different colors, then bit maps are generated for each print head. The print heads overlay the images defined by their respective bit maps onto the print medium.
To produce the bit maps for the print heads, numerous transformations must be performed on a print image, which may include different types of data, such as line art, e.g., text and graphics, and continuous tone (contone), e.g., images. This image processing may include operations such as data compression, color space conversion, and halftoning when generating the raster bit map to print. After dissecting a print image into different components, such as color components, text art, contone, images, etc., the different elements must be merged together so that the original image, previously split into different components, is reconstructed.
Digital halftoning, also referred to as spatial dithering, is the method of rendering the illusion of continuous-tone pictures on displays that are capable of producing only binary picture elements. Many printers cannot print 256 levels of gray. For example, an electrophotographic (EP) printer may only be able to print a black dot or no dot. Other printers can print 8 or 16 levels at one spot, but no more. However, it is desirable to print using 128-256 levels of gray to ensure that images look good. Halftoning takes a gray area and replaces it with a pattern of dots, much like what is done for newspaper pictures. The size of the dots in an area determines the macroscopic appearance of gray levels. Large black dots tend to make the area look darker, while small dots tend to make the area look lighter.
A pel is the smallest dot that can be displayed by an output device. In an EP printer, this is the size of a single dot drawn on the drum by the laser. Note that in the EP process, a single, isolated pel may not be drawn reliably because it is so small. This is a result of electrostatics. In an inkjet printer, a pel is the size of a drop of ink on the paper. A spot is a shape drawn in pels for halftoning. Rather than darkening a random pixel for every increase in gray level, it is preferable for the pixels to be darkened in a specific order pursuant to a “spot function”. A spot may contain one or more pels. For example, in inkjet printing a single pel can be printed fairly reliably and therefore a spot for inkjet may be a single pel. In EP, because a single pel might not be printed reliably, a spot generally consists of several pels together. A cell is a tile region which can contain one or more spots. For example, a cell may contain one spot at the center and a quarter of each of 4 spots at the corner, although this depends on the design of the spot. The image is tiled with these cells to create the halftoned image. For spot functions utilizing threshold matrices, the pels or cell elements for which thresholds are exceeded are printed as black while the remaining elements are allowed to remain white. The human eye integrates the distribution of white and black over the cell as gray. In this manner, there can be gradual transitions from different shades of gray among adjacent halftone cells.
For color applications, several halftone cells, each corresponding to a different color component, are formed for a given area. The color system superimposes the halftone cells of different color components to form the desired color of the image. Halftoning facilitates varying the concentration or intensity level of color components within the color image by varying the number of darkened pixels for halftone cells corresponding to particular color components. In this manner, the image can have transitions between neighboring colors among adjacent halftone cells.
In the interest of speed, it is often desirable to store the pixels' representation of the spot function in memory for later use. To do so, the spot function is evaluated at the location of each pixel in the cell, the pixels are rank ordered according to their respective spot function values, and a threshold value representing 0% to 100% is assigned to each pixel according to its rank. Each pixel has an associated “threshold value” which is equal to the gray level at which that pixel is darkened. If a spot which represents a gray level of 75% is desired, the spot is created by darkening every pixel with a threshold value of 75% or less. A single collection of pixels with threshold values representing a spot function is defined herein as a “halftone cell” or “cell” as defined earlier above. The terms “spot function” and “cell” are often interchangeable, as a cell is a quantized representation of a spot function, and the threshold values of the pixels map the spot function.
In this manner, a “digital screen” is created. Image processing apparatus and processes are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another measure of image processing apparatus and process merit is the tendency to produce visual artifacts in the output image that are not part of the original image, but are the result of the image processing. A conventional circular spot function consists of a spot which grows until it touches its neighbors in four places in the midtone region. This causes a change in dot gain near these regions, resulting in artifacts. Furthermore, the conventional circular spot creates awkwardly shaped white spots in regions with dark halftoning, aggravating dot gain. Mottling occurs on some high-speed printers when the spot function shape “stresses” the printer. Artifacts are present in midtone regions with some other spot functions. These artifacts are caused by the touching of the spot functions, and the resulting dot gain.
It can be seen that there is a need for a spot function that creates halftones that possess pleasing shapes in both light and dark regions, while controlling the manner in which growing spots touch in the intermediate gray regions.
It can also be seen that there is a need for a method and apparatus that provides a spot function that provides a more circular shape in the lighter and darker regions, reduces the stress on the printer, and reduces mottling.
It can also be seen that there is a need for a method and apparatus that provides more control over the position and severity of the touching of spot functions in the midtone regions.